The pelagic zone is the open ocean’s water column, which extends from the surface to the bottom. Another name for it is the wide ocean. The phrase “open sea” (pélagos) in Ancient Greek is where the word “pelagic” originates. The five depth zones that make up the pelagic zone each have unique marine life and environmental characteristics.
The pelagic zone consists of pelagic zone animals and plants. In this article, we will study the definition of the pelagic zone, the different layers of the pelagic zone, the flora and fauna found in the pelagic zone, the human impact, and its conservation.
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Definition of the Pelagic Zone
The Pelagic Zone is the water column that constitutes the vast oceans/lakes that are not in contact with the shores (littoral zone) or the bottom (benthic zone) and are far from the continental shelf.
What is Pelagic Zone?
The Pelagic zone also called the Oceanic Zone (for oceans) or the Limnetic Zone (for lakes and ponds), is part of the open water mass not associated with any landmass. It is the water column that constitutes the vast oceans and lakes that are not in contact with the shores (littoral zone) or the bottom (benthic zone) and are far from the continental shelf.
It is the largest inhabited ecosystem in the world. With a volume of 1,370,000,000 cubic kilometres (330,000,000 cubic miles) and a vertical range of 11,000 metres (36,000 feet), it is the largest inhabited environment on Earth.
Diagram of Pelagic Zone
The labelled diagram of pelagenic zone with their sub zones are shown below:
Sub-Zones of Pelagic Zone
The Pelagic zone can be divided into five sub-zones according to their depths. These are as follows:
Epipelagic Zone
The Epipelagic zone is the first layer of the Pelagic zone and can be characterized as follows:
- This uppermost layer extends from the ocean’s/lake’s surface to about 200m.
- The epipelagic zone is also called the sunlight of the surface zone, as it receives the most sunlight, allowing for photosynthesis and supporting a high concentration of phytoplankton.
- It is also home to most oceanic/pond/lake biodiversity, including plankton, surface plants, fish, sharks, dolphins, and marine birds.
- Epipelagic waters are generally warmer than deeper layers, and the wind actively mixes the water in these layers, keeping it warm.
- It is commercially the most important layer, harboring a significant portion of the Earth’s edible fish stocks.
Mesopelagic Zone
The features of the mesopelagic sub-zone can be summarized as follows:
- The zone of the oceanic water column between 200-1000 meters is known as the mesopelagic zone.
- It is also known as the Twilight Zone, as it receives reduced sunlight, creating a dimly lit environment.
- In adaptation to dimly-lit conditions, marine life usually consists of organisms with large eyes like Squids and Octopuses.
- As the mesopelagic zone receives little light photosynthesis is reduced, leading to a reliance on organic material falling from upper layers.
- There is a steep decrease in the temperature of water at the end of the epipelagic zone and in the mesopelagic zone called the thermocline.
Bathypelagic Zone
Bathypelagic sub-zone is the largest layer among the five subzones of the Pelagic Zone. It has the following characteristics:
- The pelagic zone between the depths of 1000-4000 meters is referred to as the Bathypelagic zone.
- As no sunlight reaches this zone, it results in a pitch-black environment and thus, this sub-zone is also called the midnight zone.
- To adapt to the darkness, many organisms exhibit bioluminescence as a form of communication, camouflage, or attracting prey.
- Species in these zones have also evolved to withstand extreme pressure conditions found at these depths, are mostly flattened, and have thick skin.
- Organisms in this zone often have slow metabolic rates to conserve energy, as lack of photosynthesis makes it a low-resource environment. It entirely depends on nutrients coming from the upper layers.
Abyssopelagic Zone
The abyssopelagic layer of the abyss is the dark, deep part of an ocean that is generally species-poor. The important features of this layer are:
- This is the layer of the pelagic zone that ranges from 4000-6000 meters deep.
- This layer is entirely dark and nutritionally poor.
- The abyssopelagic zone experiences intense water pressure due to its depth, with organisms adapted to withstand these conditions.
- Water temperatures in this region is consistently near freezing point.
- Very few organisms are found in these depths. Those that are found in this depth have unique adaptations, such as transparent bodies, bioluminescence, or elongated shapes, to thrive in this challenging environment.
Hadalpelagic Zone
The bottom layer of a deep ocean, which includes the water found in trenches and other sub-oceanic landforms, consists of this layer. It can be described as:
- This extends from below 6,000 metres to the ocean floor.
- Deep oceanic trenches, such as the Mariana Trench, and caves represent the deepest parts of the ocean.
- This layer experiences the highest water pressure and extremely low temperatures just above the freezing point of water.
- Due to the different landforms at the bottom, this zone has distinct geological formations.
- The Hadalpelagic Zone harbours limited but specialised extremophiles and adapted organisms, especially organisms that rely on chemosynthesis, which allows them to use chemical energy from the seafloor rather than photosynthesis, to produce food.
Flora of the Pelagic Zone
The pelagic zone of a water body is not connected to any landmass and thus there are no rooted plants present here. Any macroscopic plants cannot survive in the deeper layers of the pelagic zone due to the lack of sunlight and nutrients. Therefore, the flora of the Pelagic Zone is predominantly microscopic cyanobacteria and floating algae like phytoplanktons, diatoms, and dinoflagellates. These serve as primary food for smaller fishes and are important in the marine food web.
Pelagic Zone Animals
As the pelagic zone can be divided into layers that differ vastly in depth, availability of sunlight, nutrition, and pressure, the life it supports in each layer is also unique and shows various degrees of adaptations.
Epipelagic Life: This is the region of life. This layer of the Pelagic zone supports all kinds of life like Phytoplankton (e.g., diatoms, dinoflagellates), Zooplankton (e.g., copepods, krill), Fish (e.g. tuna, mackerel), sharks, dolphins as well as marine birds like seagulls that rest on the surface and feed on small fishes.
Mesopelagic Life: This layer, due to lack of sunlight, starts showing distinctive adaptations to light, like larger eyes and bioluminescence. Animals found are Lanternfish, Squids, eels, dragonfish, etc.
Bathypelagic Life: Most of the organisms in this layer are adapted to darkness, have slow metabolism to counter the lack of nutrients, and can survive in extreme pressure and cold temperatures. E.g.- anglerfish, giant squid, gulper shark (e.g., Centrophorus granulosus), deep-sea jellyfish (e.g., Atolla wyvillei), etc.
Abyssopelagic Zone: The number of organisms drastically reduces in this layer due to lack of nutrition, and they reproduce slowly. Characteristic organisms include basket star, deep-sea octopus (e.g., Graneledone boreopacifica), sea cucumbers, snailfish, abyssal grenadier, etc.
Hadalpelagic Life: Although this is the bottom of the ocean. Life still exists here. It includes mostly shelled organisms that can bear intense pressure, like amphipods, giant isopods, snailfishes, deep-sea anemones, and worms.
Human Impact and Conservation
Although the pelagic zone is far away from any landmass, it is not immune to human impact. Overfishing, pollution, climate changes, oil spills, coral bleaching, sea bed mining, and increased water tourism are known to negatively impact the pelagic zone and all the creatures that live in it.
Sustainable measures are thus necessary to protect these areas. Conservation efforts have been made to protect the pelagic zone by introducing Marine Protected Areas, regulating fishing, and also reducing pollution from adjacent landmasses. Proper research and awareness are also important to help protect the pelagic zone and its flora and fauna.
Conclusion: Pelagic Zone
The Pelagic Zone, with its distinct layers, thus fosters a vast expanse of countless species from microscopic phytoplankton shaping the foundation of the food web to the elusive giants of the deep, like the mysterious giant squid. The Pelagic zone represents more than just a giant stretch of water as it is home to an unknown amount of organisms, some yet to be discovered, and thus holds the key to understanding life on our planet.
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FAQ’s – Pelagic Zone
What do you mean by a Pelagic Animal?
An organism that lives in the pelagic zone of the ocean. The pelagic zone refers to the part of the ocean that does not touch any land, and all animals that live in these free waters are called pelagic animals.
What is the Difference between Pelagic and Benthic ?
The pelagic zone is the water column where swimming and floating organisms live. The benthic zone represents the bottom sediment surface of the ocean and is home to bottom dwellers.
Name the Five Pelagic Zones.
The five pelagic zones are: the upper most Epipelagic, Mesopelagic, Bathypelagic, Abyssopelagic and the bottom Hadalpelagic zones.
What is the Depth of the Pelagic Zone?
The pelagic zone ranges from the surface of the water to the bottom of the ocean or ocean floor. At places it can be a few 1000 m at others it can go up to 10,000 m.
What is the Largest Ocean Zone?
The largest oceanic zone is the pelagic zone.It includes all life forms found in these waters and is the largest ecosystem of the planet.